1. Field of the Invention
The present invention generally relates to wireless communications, and more particularly to a direct-sequence spread-spectrum (DSSS) packet receiver that iteratively removes precursor inter-symbol interference (ISI) as well as postcursor ISI in wireless multipath channels by employing a bidirectional turbo ISI canceller.
2. Description of the Related Art
DSSS techniques have widely been applied to major wireless communications standards such as IS-95, CDMA 2000, W-CDMA, IEEE 802.11 wireless local area networks (WLAN) as well as others. These DSSS systems have in common transmitting information bits in the form of wideband chip sequences, but they have noticeable differences in terms of embedded data modulation methods, i.e., from the most popular BPSK and QPSK to the higher-order QAM and the codeword selection modulations.
The code complementary keying (CCK)-based DSSS system has recently been adopted as a 5.5 Mbps/11 Mbps rate 802.11b standard for higher speed extension of the original 1 Mbps/2 Mbps rate 802.11 WLAN. The 802.11b DSSS/CCK is a codeword selection modulation based spread spectrum method which can transmit 4 or 8 information bits per symbol using an 8-long quaternary complementary codeword set of size 256.
Because CCK codewords have good auto- and cross-correlation properties, a typical maximal ratio combining (MRC) RAKE receiver and codeword correlator bank can achieve satisfactory decoding performance in additive Gaussian or moderate multipath channels. However, as the multipath delay spread becomes larger than 100 ns, the simple RAKE-based correlator bank cannot sustain this performance any longer and thus it is necessary to complement the receiver with complex signal processing such as equalization of ISI or inter-chip interference (ICI).
To perform MRC processing and equalization, the receiver needs to know accurate multipath channel coefficients. In a typical wireless LAN environment where the multipath channel changes very slowly and its profile is closely spaced, the channel coefficients are usually estimated in advance using a wideband preamble sequence part and the estimates are used to decode a data symbol part in the same packet. In this case, the MRC RAKE receiver can be implemented in the form of a channel matched filter (CMF) whose taps are the conjugate of the time-reversed channel estimates, and it is usually placed in front of the correlator bank in order to minimize the receiver implementation complexity by completing the multipath combining before codeword correlation. The CMF not only provides a multipath diversity advantage but also warrants robust time-tracking during the data symbol detection, as the signal has a symmetrical shape around a real-valued central peak after passing through the CMF.
On the other hand, if a decision feedback equalizer (DFE) is employed between the CMF and the codeword correlator bank to improve the detection performance, the associated DFE coefficients should also be estimated using the preamble part. In the case that only a feedback filter is employed that cancels a postcursor-ISI, the DFE coefficients can be directly calculated through a simple auto-correlation of the CMF coefficients. However, if a feedforward filter is incorporated to further improve performance by suppressing precursor-ISI, receiver complexity significantly increases because one or more of a complex matrix inversion, spectral factorization, or adaptive equalizer training must also be applied.
A feedforward filter requires complex multiplication operations in data detection processing, while a feedback filter needs only addition operations. Thus, in practical high-speed WLAN packet transmissions, the DFE is usually composed of only a feedback filter without the capability of precursor-ISI suppression. When the multipath delay spread becomes long, the remaining precursor-ISI critically affects the packet decoding performance.
In view of the foregoing, a need exists for a practically feasible precursor-ISI cancellation system and method, and more particularly one which may be used in a variety of communications systems including but not limited to a DSSS packet receiver.